The present invention relates to the art of petroleum production in general, and, more in particular, to power fluid purification systems for a power fluid used downhole in a production well.
Petroleum wells quite often employ a power fluid. The fluid may operate a motor which in turn operates a downhole pump that provides sufficient head to raise petroleum values to the surface. In such a scheme, it has been convenient to use as a power fluid produced petroleum or produced water.
The effectiveness of the power fluid is directly related to the abrasives content. Small, solid particles in the power fluid can score and damage power fluid circuit machinery. For example, seals of a downhole pump motor can be lost, with a loss of effectiveness of the motor in producing the requisite power for a downhole pump to pump the petroleum. Also, damage to downhole machinery requires the lifting of the machinery from the well for repair or renewal. Clearly, both of these results are not satisfactory. In the first place, loss of the effectiveness of the motor shuts down the well. Second, it takes a considerable amount of time to raise and lower machinery and this time means loss of production. Moreover, it is expensive to lift and replace downhole machinery.
The power fluid mixes with production fluid in the petroleum recovery zone of the well and this admixture of production fluid in the power fluid is the source of the abrasives.
The art recognizes these problems and has proposed several approaches to maintain power fluid adequately free of abrasives.
One technique that has been used for cleaning power fluid employs one or more cyclone centrifugal separators. These separators, because of differences in density of the constituent parts of the fluid, separate out heavier particulates from lighter, purified liquid by centrifugal force. Examples of this technique are described in U.S. Pat. No. 3,709,292 to Palmour and U.S. Pat. No. 3,802,501 to Mecusker.
Cyclone separators are sensitive to the proportion of dirty fluid withdrawn from them. The flow rate to the cyclones and from the cyclones must be carefully controlled. There also must be a careful balance of the operating pressures that the cyclone experiences, namely the inlet and two outlet pressures. Differences in operating conditions can adversely affect the effectiveness of the cyclone. For example, differences in the proportion of the phases in the production fluid can result in an unacceptably high solid content in the stream from the cyclone for purified fluids.
One approach to avoiding the problem of the cyclone is disclosed in U.S. Pat. No. 3,982,589 to Wilson et al. This patent uses a separation vessel for the initial separation of the phases of the production fluid. A pitot pump and cleaner further purifies an effluent from the separation tank and produces power fluid for a multiplex pump that is used to raise the head of the power fluid.
In some well applications the production fluid is at a high pressure and it is undesirable to lose this pressure. The pressure can be used, for example, to force product fluid from the production fluid through surface lines. The use of settling tanks, however, to produce power fluid of adequate purity in a pressurized environment is not practical because the cost of the pressure vessel becomes too high.
It is desirable, then, to have a production fluid purification system that enables the separation of solids from production fluid while maintaining the pressure head of the production fluid so that the head may be used to force product fluid through surface lines. In addition, in such a system, it is desirable to form a separate stream of power fluid. This facility should avoid the problems attendant with cyclone separation.